1. Field of the Invention
The present invention relates to an image forming apparatus such as copying apparatus and printer having a function of forming an image onto a recording medium such as a sheet and, more particularly, to an image forming apparatus having a contactless type temperature sensor.
2. Description of the Related Art
Hitherto, in an image forming apparatus, a toner image formed on a sheet is thermally fixed onto a sheet surface by using a fixing unit (fixing roller, fixing belt) having a heat source. Since a management of a fixing temperature in this instance is very important to stabilize sheet conveying performance and image quality, generally, proper heating control is made to the heat source while monitoring a temperature of a surface of the fixing unit by using a temperature detecting sensor.
Hitherto, a contact type sensor such as a thermistor has widely been used as a temperature detecting sensor.
However, if the contact type sensor is used, since surface states (temperature distribution and a consumption degree) of the fixing roller and the fixing belt, particularly, only regions which come into contact with the sensor change, such a change is liable to appear as a gloss variation on the image fixed onto the sheet. Since micro toner foulings deposited onto the fixing roller and the fixing belt are accumulated into the contact type sensor, there is such a problem that when the accumulated fouling is suddenly peeled off, it is deposited onto the fixed image.
Since it is necessary to avoid those problems in order to realize high picture quality of the image forming apparatus, a contactless type sensor such as a thermopile is becoming a main stream.
However, since the contactless type sensor is very expensive, if it is built in the fixing unit which is presumed to be exchanged as consumables, it directly exerts an influence on the costs of the consumables. Since such a sensor is a contactless type, it has to be arranged at a predetermined distance from the surface of one of the fixing roller and the fixing belt. Therefore, when the sensor is built in the fixing unit, it is necessary to assure a certain space into the fixing unit.
From the above circumstances, as a construction of the image forming apparatus, generally, the contactless type sensor is provided on the image forming apparatus main body side and the temperature of the fixing unit is detected through an opening portion for temperature detection provided for a cover of the fixing unit.
As literatures disclosing the related arts, there are Japanese Patent Application Laid-Open Nos. 2001-228742, 2002-296953, and 2003-098866.
FIG. 5 illustrates a conventional construction in which a contactless type sensor 151 is provided on the main body side of an image forming apparatus 150 and a temperature of a fixing unit 140 is detected through an opening portion for temperature detection 144 provided for a cover 143 of a fixing unit 125.
In such a conventional construction, there is a case where the temperature detection is influenced by a flow (shown by an arrow in FIG. 5) of the air around the fixing unit.
Such a problem occurs because the low-temperature air out of the fixing unit flows from the temperature detecting opening portion 144 provided for the cover 143 of the fixing unit and the surface temperature of the fixing unit 140 is locally cooled. A controller of the image forming apparatus makes the heating control to the heat sources 142 of the fixing unit so as to maintain a temperature of a cooled detection area 140a to a target control temperature. Therefore, temperatures of areas other than the detection area exceed the target temperature. The over-temperature of the fixing unit becomes a cause of winding of a sheet or an over-gloss of the picture quality. A temperature difference between the detection area and the other areas becomes a cause of the gloss variation of the fixed image.
Generally, there is a tendency that a flow occurs in the air around the fixing unit by a heat convection. Especially, in the construction in which the contactless type sensor is arranged under the fixing unit, an influence of the heat convection from the temperature detecting opening portion is liable to occur. In dependence on the apparatus construction, a heat insulating air flow is often formed near the fixing unit in order to protect an image forming portion adjacent to the fixing unit and a laser scanner from the heat. In such a case, the air flows easily from the temperature detecting opening portion into the fixing unit in terms of such characteristics of the air that flows in the direction of a lower atmospheric pressure. The problem of deterioration in temperature detecting precision appears further typically.
As a conventional method of avoiding the foregoing problems, as illustrated in FIG. 6, a method whereby a window 145 made of an infrared transmitting material is attached to the temperature detecting opening portion 144 of the fixing unit 125 and the inflow of the air is prevented has been known.
However, since transmittance of infrared rays of the window 145 made of the infrared transmitting material changes depending on the surface state, there is a risk that the detecting precision becomes unstable due to a factor such as variation among parts or deposition of the fouling. When the sheet passes through the fixing unit 125, there is a case where the inner surface of the window 145 forms dew and becomes cloudy by an influence of the moisture evaporated from the sheet by the heat. Thus, there is also a risk that the temperature cannot be detected but an abnormality occurs in the heating control and, in the worst case, a temperature of the fixing unit exceeds a limit temperature and the fixing unit is broken.